In recent years, high-pressure mercury lamps have been widely used as a light source for liquid crystal panels, DLP (Digital Light Processing: Registered Trademark) projection-type display apparatuses and the like.
For example, the above-mentioned high-pressure mercury lamp (hereinafter, simply referred to as “lamp”) is composed of a bulb made of a quartz glass, including therein a pair of tungsten electrodes and enclosing a rare gas such as mercury or argon and a halogen such as bromine. The pair of tungsten electrodes are disposed opposing each other with a predetermined gap in between.
Also, the above lighting apparatus normally performs a control on a lamp voltage and a lamp power such that a control characteristic is as shown in FIG. 13. That is to say, when the lamp voltage is lower than a first voltage value V1, a constant current control is performed so as to increase the lamp voltage and power linearly; when the lamp voltage is equal to or higher than the first voltage value V1, a constant power control is performed to maintain the lamp power at a power value W1. Note that a point at which the lamp voltage is equal to the first voltage V1 and the lamp power is equal to the power value W1 is, in other words, a switching point at which the constant current control is switched to the constant power control.
On the other hand, in the lamp, a protuberance is formed and grows at a tip of the opposing electrodes in the bulb (for an example, refer to Patent Document 2) as operating time (in the present specification, “operating time” is a period of time during which a discharge of a lamp is maintained) increases. Formation of such a protuberance shortens a distance between the electrodes, lowering the lamp voltage as a result.
Moreover, when the lamp voltage lowers further as the discharge of the lamp is maintained and the operating time progresses further, the lamp voltage becomes lower than the first voltage value V1, the switching criterion between the constant current control and the constant power control, resulting in a change of the control from the currently-performed constant power control to the constant current control.
As a result, an optical output (illuminance) may decline due to a lack of power applied to the lamp, and a life of the lamp may be shortened by a so-called “blackening phenomenon” caused by a fall in a lamp temperature also due to the lack of power.
In attempts to solve these problems, some techniques have been disclosed. For example, one such technique changes an operating frequency to supplement the fall of the lamp voltage when the lamp voltage falls due to the formation of a protuberance (Patent Document 1). Another such technique lowers the operating frequency of the lamp to raise the lamp voltage when the lamp voltage falls below the first voltage value V1, the switching point from the constant current control to the constant power control (Patent Document 2).
Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2001-312997
Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2004-172086